2^x+2^y>=2*√2^(x+y)
而x+y=x+1/2*x^2=1/2(x+1)^2-1/2
所以x+y的最小值为-1/2
2^x+2^y>=2√2^(x+y)>2*√2^(-1/2)=2^(3/4)
所以log2（2的x次+2的y次）＞3/42*√2^(-1/2)=2^(3/4)这一步最后是怎么出来的2*√2^(-1/2)=2^(1-1/2*1/2)=2^(3/4)